Regulatory Capital - Summary of the Bank's Compliance (Details) $ in Thousands	Dec. 31, 2022 USD ($)	Dec. 31, 2021 USD ($)
Savings Bank [Member]
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 915,628	$ 840,105
Tier I (leverage) capital, capital level, percent	0.1056	0.1039
Tier I (leverage) capital, requirement to be well capitalized, amount	$ 433,667	$ 404,366
Tier I (leverage) capital, requirement to be well capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 481,961	$ 435,739
Tier I (leverage) capital, excess, percent	0.0556	0.0539
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 915,628	$ 840,105
Common Equity Tier I risk-based capital, capital level, percent	0.1379	0.1358
Common Equity Tier I risk-based capital, requirement to be well capitalized, amount	$ 431,734	$ 402,100
Common Equity Tier I risk-based capital, requirement to be well capitalized, percent	0.065%	0.065%
Common Equity Tier I risk-based capital, excess, amount	$ 483,894	$ 438,005
Common Equity Tier I risk-based capital, excess, percent	0.0729%	0.0708%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 915,628	$ 840,105
Tier I risk-based capital, capital level, percent	0.1379	0.1358
Tier I risk-based capital, requirement to be well capitalized, amount	$ 531,365	$ 494,892
Tier I risk-based capital, requirement to be well capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 384,263	$ 345,213
Tier I risk-based capital, excess, percent	0.0579	0.0558
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 954,457	$ 874,400
Total risk-based capital, capital level, percent	0.1437	0.1413
Total risk-based capital, requirement to be well capitalized, amount	$ 664,206	$ 618,615
Total risk-based capital, requirement to be well capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 290,251	$ 255,785
Total risk-based capital, excess, percent	0.0437	0.0413
Holding Company
Tier I (leverage) capital:
Tier I (leverage) capital, capital level, amount	$ 746,880	$ 726,174
Tier I (leverage) capital, capital level, percent	0.0861	0.0898
Tier I (leverage) capital, requirement to be well capitalized, amount	$ 433,607	$ 404,422
Tier I (leverage) capital, requirement to be well capitalized, percent	0.0500	0.0500
Tier I (leverage) capital, excess, amount	$ 313,273	$ 321,752
Tier I (leverage) capital, excess, percent	0.0361	0.0398
Common Equity Tier I risk-based capital:
Common Equity Tier I risk-based capital, capital level, amount	$ 698,258	$ 671,494
Common Equity Tier I risk-based capital, capital level, percent	0.1052	0.1086
Common Equity Tier I risk-based capital, requirement to be well capitalized, amount	$ 431,635	$ 401,836
Common Equity Tier I risk-based capital, requirement to be well capitalized, percent	0.065%	0.065%
Common Equity Tier I risk-based capital, excess, amount	$ 266,623	$ 269,658
Common Equity Tier I risk-based capital, excess, percent	0.0402%	0.0436%
Tier I risk-based capital:
Tier I risk-based capital, capital level, amount	$ 746,880	$ 726,174
Tier I risk-based capital, capital level, percent	0.1125	0.1175
Tier I risk-based capital, requirement to be well capitalized, amount	$ 531,243	$ 494,568
Tier I risk-based capital, requirement to be well capitalized, percent	0.0800	0.0800
Tier I risk-based capital, excess, amount	$ 215,637	$ 231,606
Tier I risk-based capital, excess, percent	0.0325	0.0375
Total risk-based capital:
Total risk-based capital, capital level, amount	$ 975,709	$ 885,469
Total risk-based capital, capital level, percent	0.1469	0.1432
Total risk-based capital, requirement to be well capitalized, amount	$ 664,054	$ 618,210
Total risk-based capital, requirement to be well capitalized, percent	0.1000	0.1000
Total risk-based capital, excess, amount	$ 311,655	$ 267,259
Total risk-based capital, excess, percent	0.0469	0.0432